Coral reefs around the world — from Hawaii to the iconic Great Barrier Reef, eastward all the way to the Bahamas and beyond — are in jeopardy of being severely damaged or even dying because of a dangerous spike in ocean temperatures, scientists say.

Conditions are so dire that, provided coral bleaching soon spreads from the Florida Keys to the Bahamas, the National Oceanic and Atmospheric Administration is expected to declare as soon as two weeks from now that the third global coral bleaching event is here. There's already evidence of coral bleaching in three major ocean basins.

“It's like watching a slow-motion train wreck, and we’re waiting for the cars to pile up on this side of the track,” says Mark Eakin, the coordinator of NOAA's Coral Reef Watch, in an interview with Mashable.

Area with at least a 60% chance of exceeding certain thermal stress thresholds in October 2015.

Image: NOAA Coral Reef Watch

The previous two such global events, in 1997-98 and 2010, devastated reefs around the world, some of which have not yet recovered. Global bleaching events are becoming more common because of global warming, with many coral reefs expected to perish by the middle of the century if recent trends continue. Other reefs, which can survive in a broader temperature range, may prove more resilient.

On maps showing the outlook for coral bleaching, this event resembles a red crocodile. The mouth of the beast is projected to bite down on the western Pacific by mid-October, devouring reefs from Hawaii to Palau. After that, even more coral bleaching is likely from the Indian Ocean to the South Pacific, as the Southern Hemisphere summer kicks in. This, the projections show, will include Australia's iconic Great Barrier Reef, which is a top tourist destination.

The cause of the unusually high ocean temperatures is a double whammy of manmade global warming, which is leading to warmer, more acidic seas, and natural climate variability, which has brought a strong El Niño event to the tropical Pacific Ocean.

For Hawaii, though, the drivers of the record warm water temperatures are more mysterious. An area of much above average ocean temperatures, stretching from California to the Hawaiian Islands, has proven to be a long-lasting, rather mysterious climate feature.

Nicknamed "The Blob," this area of mild waters has been implicated in enhancing California's drought.

Richard Vevers, executive director of the XL Catlin Seaview Survey, says he saw corals that were starting to bleach in Kaneohe Bay on Oahu while scuba diving in recent weeks.

Coral bleaching occurs when waters reach certain temperatures, causing corals to expel the algae that live in their tissues and give them their vibrant colors, while supporting extraordinarily diverse ecosystems. When a coral bleaches, it exposes its skeleton and turns white. It is then subject to higher mortality rates from further heat stress or the effects of pollution.

Sea surface temperature anomalies for mid-September 2015.

Image: NOAA/NNVL

Vevers is part of an ambitious effort to document the world's reefs through high-resolution photography, in partnership with Google and others. So far, the survey has observed reefs in 26 countries, establishing valuable scientific baselines for bleaching events like this one and extending Google's .

Vevers says the big difference between this bleaching event and the previous two global events, which occurred in 1997-98 and again in 2010, is that this one is being closely tracked by firsthand images from his group and NOAA's predictive tools.

Regarding Hawaii specifically, Vevers says, "it’s unprecedented the amount of heat that will be sitting on the main islands."

"It is predicted to be quite devastating in Hawaii,” Vevers said.

A major bleaching event could be a big hit to the state's economy. About half of the visitors to Hawaii participate in water activities — including snorkeling and scuba diving — and the islands and their surrounding waters are home to 85% of the coral reefs in the U.S. This event comes on the heels of the first documented mass bleaching event there, which occurred just last summer.

The global average ocean temperature during August was 1.40 degrees Fahrenheit, or 0.78 degrees Celsius, above the 20th century average, which was the highest temperature for any month on record since such data began in 1880, NOAA found. This beat the previous record, set just one month earlier, by 0.07 degrees Fahrenheit, or 0.04 degrees Celsius.

Sea surface temperature anomalies from 1880 through August 2015, showing the peak during August.

Image: NOAA NCEI

“The models are indicating that it’s going to last pretty much through the end of the year. For Hawaii in particular, it’s going to last until November,” Eakin told Mashable. Eakin and his colleagues have raised the coral bleaching alert level in and around the Hawaiian Islands to a level two, which is the highest category.

“Once it gets to that point, it’s bad,” Eakin said.

A NOAA research vessel recently explored a marine national monument in northwestern Hawaii, where there was severe coral bleaching in 2014. “We did see signs of recovery of corals in certain places,” said Courtney Couch, a researcher with the Hawaii Institute of Marine Biology, in a press release. “But at certain sites that experienced the most severe coral bleaching, we found that 85-100% of the corals have died. This is especially concerning since these sites are home to some of the most unique and rare coral species.”

In general, the duration of a bleaching event affects reef mortality rates, with longer events proving more deadly.

Lisianski Atoll, which is in the northwestern part of Hawaii's waters, had about 18 degree weeks of thermal stress last year, Eakin says. For perspective, alert level two kicks in at eight weeks.

He said such bleaching, which is likely for the main Hawaiian Islands and southern parts of the islands' waters this year, was equivalent to “All but boiling corals.”

360 degree images of corals

Antilles Shipwreck, Aruba

Image: Catlin Seaview Survey

A before and after image of the bleaching in American Samoa. The first image was taken in December 2014. The second image was taken in February 2015 when the XL Catlin Seaview Survey responded to a NOAA coral bleaching alert.

Image: XL Catlin Seaview Survey

Green Turtles, Heron Island, Australia

Image: Catlin Seaview Survey

Oceanographic Museum of Monaco

Image: Catlin Seaview Survey

SVII surveys the Coral Nursery, FKNMS, Florida

Image: Catlin Seaview Survey

A marine biologist assesses the bleaching at Airport Reef in American Samoa.

Image: XL Catlin Seaview Survey

Mary Celeste Wreck, Bermuda

Image: Catlin Seaview Survey

Eagle Ray, Long Cay, Belize

Image: Catlin Seaview Survey

A marine biologist assesses the bleaching in Kaneohe Bay in Hawaii during Oahu’s first ever mass bleaching event in late 2014.

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